Paraxylene (PX), which is separated from metaxylene (MX) and orthoxylene (OX) on a commercial scale typically by subzero crystallization separation or simulated moving bed adsorptive separation, has widespread applications in many fields such as chemical fiber, synthetic resin, pesticide, medicine, plastics, and so forth. The use of disproportionation of toluene, or the disproportionation and transalkylation of toluene and C9+ aromatics to produce benzene and C8 isomers (including xylenes and ethyl benzene) are effective routes for increasing the output of PX. Examples of these methods are described, for instance, in U.S. Pat. No. 6,528,695.
A source of xylene is catalytic reformate. Catalytic reformate is typically prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. A C6 to C8 fraction can be separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains benzene, toluene and xylenes (collectively “BTX”), along with ethylbenzene. The separation of this mixture into its constituent parts to upgrade the value thereof has been and still is the subject of continuous research.
Xylene is also available from the transalkylation of C9+ aromatics and toluene over noble metal-containing zeolite catalysts. During the transalkylation of C9+ aromatics and toluene to high value petrochemical products, by-products, such as saturated compounds, are typically produced. These by-products can boil in the same temperature range as the desired aromatic products, making separation of the desired products at high purity levels difficult. For example, a commercial benzene product may need a purity of 99.85 wt % or higher. However, initial benzene purity after distillation of a transalkylation reaction product is typically only 99.2 wt % to 99.5 wt % due to the presence of co-boilers, such as methylcyclopentane, cyclohexane, 2,3-dimethylpentane, dimethylcyclopentane and 3-methylhexane. Therefore, an additional extraction step is usually required to further improve benzene product purity to the desired level.
One solution to the problem of the production of benzene co-boilers during the transalkylation of heavy aromatics is disclosed in U.S. Pat. No. 5,942,651 and involves the steps of contacting a feed comprising C9+ aromatic hydrocarbons and toluene under transalkylation reaction conditions with a first catalyst composition comprising a zeolite having a constraint index ranging from 0.5 to 3, such as ZSM-12, and a hydrogenation component. The effluent resulting from the first contacting step is then contacted with a second catalyst composition which comprises a zeolite having a constraint index ranging from 3 to 12, such as ZSM-5, and which may be in a separate bed or a separate reactor from the first catalyst composition to produce a transalkylation reaction product comprising benzene and xylene. A benzene product having a purity of at least 99.85% may be obtained by distilling the benzene from the transalkylation reaction product, without the need for an additional extraction step. According to the '651 patent, the second catalyst composition comprises up to 20 wt. % of the total weight of the first and second catalyst compositions.
U.S. Pat. No. 5,905,051 discloses a process for converting a hydrocarbon stream such as, for example, a C9+ aromatic compound to C6 to C8 aromatic hydrocarbons, such as xylenes, by contacting the stream with a catalyst system comprising a first catalyst composition and a second catalyst composition, wherein said catalyst compositions are present in separate stages and are not physically mixed or blended and wherein said first catalyst composition is a metal-promoted, alumina- or silica-bound zeolite beta, and said second catalyst composition is ZSM-5 having incorporated therein an activity promoter selected from the group consisting of silicon, phosphorus, sulfur, and combinations thereof. According to the patent, the use of the separate catalytic stages improves the conversion of C9+ aromatic compounds and naphthalenes to xylenes and decreases the amount of undesirable ethylbenzene in the product.
U.S. Pat. No. 5,030,787 discloses an improved disproportionation/transalkylation process. The improved process of this invention is conducted such that transalkylation of a C9+ aromatics feedstock, or disproportionation of a feedstock containing toluene and C9+ aromatic(s), is carried out in the vapor-phase by containing said feedstock in a reaction zone with a catalyst comprising a zeolite possessing a Constraint Index, as defined below, of from 1 to about 3 and preferably which has been hydrogen, hydrogen precursor and/or non-noble Group VIII metal exchanged, thermally treated and/or hydrothermally treated, under conditions effective to convert such feedstock to a product containing substantial quantities of C6-C8 aromatic compounds, e.g. benzene and xylene(s), especially the latter. The product effluent is separated and distilled to remove the desired products. If desired, any unreacted material(s), e.g., toluene and/or C9+ compound(s), can be recycled.
U.S. Pat. No. 5,030,787 discloses a transalkylation process to convert a heavy aromatics feed to lighter aromatics products, such as benzene, toluene and xylenes by contacting a C9+ aromatics fraction and benzene and/or toluene over a catalyst comprising a zeolite, such as ZSM-12, and a hydrogenation component, preferably platinum. The catalyst, with hydrogenation component, is treated to reduce aromatics loss. Treatment includes exposure to steam and/or sulfur after incorporation of the hydrogenation component. For additional stability and aromatics retention, the steamed and/or sulfur treated catalyst is sulfided by cofeeding a source of sulfur. In a further embodiment of the invention, a low hydrogen partial pressure is employed to retain aromatics.
U.S. Pat. No. 7,148,391 discloses a single stage catalyst system comprising at least two different molecular sieves that exhibits enhanced activity for the removal of ethyl-group containing aromatic compounds in C9+ aromatic feeds without overall reduction in the conversion of the C9+ feed to useful compounds, such as xylenes.
U.S. Pat. No. 7,485,763 teaches a process for converting C9+ aromatic hydrocarbons to lighter aromatic products, comprising the step of contacting a feed comprising C9+ aromatic hydrocarbons under transalkylation reaction conditions with a catalyst composition comprising: (i) a first molecular sieve selected from the group consisting of ZSM-12, mordenite and a porous crystalline inorganic oxide material having an X-ray diffraction pattern including d-spacing maxima at 12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07; and (ii) a second molecular sieve having a constraint index ranging from 3 to 12, wherein at least the first molecular sieve has a hydrogenation component associated therewith and wherein the first and second molecular sieves are contained in the same catalyst bed, the C9+ aromatic hydrocarbons being converted under said transalkylation reaction conditions to a reaction product containing xylene. In one embodiment, the first molecular sieve is ZSM-12 and the second molecular sieve is ZSM-5.
U.S. Pat. No. 7,553,791 teaches a process for the conversion of a feedstock containing C9+ aromatic hydrocarbons to produce a resulting product containing lighter aromatic products and less than about 0.5 wt % of ethylbenzene based on the weight of C8 aromatics fraction of said resulting product, said process comprising contacting said feedstock under transalkylation reaction conditions with a catalyst composition comprising: (i) an acidity component having an alpha value of at least 300; and (ii) a hydrogenation component having hydrogenation activity of at least 300, the C9+ aromatic hydrocarbons being converted under said transalkylation reaction conditions to a reaction product containing xylenes. Preferably, the aromatic product contains less than about 0.3 wt % of ethylbenzene based on the weight of C8 aromatics fraction of said resulting product. More preferable, the aromatic product contains less than about 0.2 wt % of ethylbenzene based on the weight of C8 aromatics fraction of said resulting product. Preferably, the acidity component comprises a molecular sieve selected from the group consisting of one or more of a first molecular sieve having a MTW structure, a molecular sieve having a MOR structure, and a porous crystalline inorganic oxide material having an X-ray diffraction pattern including d-spacing maxima (Å) at 12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07. More preferably, the catalyst comprises a molecular sieve ZSM-12. Alternatively, the porous crystalline inorganic oxide material is selected from the group consisting of one or more of MCM-22, PSH-3, SSZ-25, MCM-36, MCM-49 and MCM-56. In another embodiment, the catalyst comprises second molecular sieve having a constraint index ranging from 3 to 12. Preferably, the second molecular sieve is ZSM-5. Preferably, the catalyst comprises two molecular sieves, the first molecular sieve is ZSM-12, and the second molecular sieve is ZSM-5. Conveniently, the catalyst composition is particulate and the first and second molecular sieves are each contained in the same catalyst particles. Preferably, the hydrogenation component is selected from the group consisting of one or more of a Group VIIIB and Group VIIB metal. More preferably, the hydrogenation component is selected from the group consisting of one or more of rhenium, platinum, and palladium.
A catalyst system for the transalkylation of C9+ aromatics with C6-C7 aromatics is disclosed in U.S. Pat. No. 7,663,010. The catalyst system described therein comprises (a) a first catalyst comprising a molecular sieve having a Constraint Index in the range of 3-12 (e.g., a 10 MR molecular sieve, such as ZSM-5, ZSM-11, ZSM-22, and ZSM-23) and a metal catalyzing the saturation of the olefins formed by the dealkylation reactions and (b) a second catalyst comprising a molecular sieve having a Constraint Index in the range of less than 3 (e.g., a 12 MR molecular sieve, such as ZSM-12, MOR, zeolite beta, MCM-22 family molecular sieve) and optionally a metal which may be the same or different to the metal on the first catalyst.
Improving catalytic activity and stability are challenges for most of the catalytic transalkylation processes. A high activity catalyst normally requires less catalyst and/or less severe reaction conditions to manufacture the same amount of product, which means lower cost for production and high production efficiency. As the catalyst ages with increasing time on stream, higher temperatures are normally required to maintain constant conversion. When the maximum reactor temperature is reached, the catalyst needs to be replaced or regenerated. Depending on the feed composition, the cycle length varies from a few months to as long as a few years for a transalkylation catalyst. A catalyst having high stability normally requires less frequent regeneration or change-out and long time on stream, which translates to lower cost for production and high production efficiency.
The present inventor has now discovered that a catalyst system comprising two beds, a first bed comprising aluminosilicates having a MWW topology, e.g. MCM-22 and MCM-49, and containing a metal, and a second bed, in series with said first bed, comprising an aluminosilicate containing a 12 MR structure, e.g. MTW (ZSM-12), that may also contain a metal, allows for processing of heavy aromatic feeds at space velocities significantly higher than competing technologies. This provides a significant advantage in that higher throughput is possible in the transalkylation unit. In addition, it has been discovered that the use of this dual bed configuration results in lower aging rates for the catalysts, thereby extending cycle lengths.
As an optional embodiment of this invention, a third bed, located just prior to the exit of the reactor, may be used to crack non-aromatic molecules, e.g. cyclohexane, to meet benzene purity specifications. The catalyst used in this optional third bed is preferably one containing an MFI zeolite.